1. Field of the Invention
The present invention relates generally to optical devices, and more particularly to cross-gain modulation type wavelength converters to change the wavelength of an incoming signal.
2. Description of the Prior Art
A wavelength converter converts a data signal at one wavelength to the same data signal at another wavelength. Wavelength converters are an important part of modern data communications networks. Wavelength converters are used when the wavelength of the carrier light wave needs to be changed at some points in the communications network.
There are two major types of wavelength converters used in digital data communications systems. The first type involves optical-electrical-optical (OEO) conversions in which the wavelength converter first converts an input light (optical) signal at an input wavelength to electrical signals, and then generates an output light (optical) signal at an output wavelength using the electric signals. Problems with OEO-based wavelength converters include high costs, complexity of electronic systems, and limited bandwidth. As the data bit rate continues to increase, the bandwidth limitation in particular is a growing barrier for OEO-based wavelength converters.
The second type of wavelength converters do not require OEO conversions but instead perform direct optical-to-optical conversions. This type of wavelength converters can generally be called optical wavelength converters. One known optical wavelength converter consists of a screen of luminescent material that absorbs radiation at a certain wavelength and radiates at a longer wavelength. Such materials are often used to convert ultraviolet light to visible radiation for detection by conventional photo tubes. This type of optical wavelength converters, however, are not highly suitable for converting wavelengths of coherent light sources. For this reason, these optical wavelength converters are more commonly used as scintillators in optical equipment such as X-ray spectroscopy than in digital data communications networks which primarily use coherent light sources (such as laser).
For coherent light sources used for carrying digital signals, wavelength conversion typically uses nonlinear optical techniques. This type of wavelength converters may generally be called nonlinear optical wavelength converters. One example of nonlinear optical wavelength converters is cross-gain modulation (XGM) wavelength converters that are based on XGM phenomena using a semiconductor optical amplifier (SOA). Because XGM is a form of optical gating, XGM wavelength converters operate based on optical gating technology. These devices use a modulated input signal to impress the data pattern onto a second laser-emitted light, which becomes a modulated output signal.
FIG. 1 is an illustration of an XGM wavelength converter in the prior art. The XGM wavelength converter has a semiconductor optical amplifier (SOA) 10. Input optical signal 12 at wavelength λin enters through an optical coupler 14 into SOA 10 from one side. A continuous wave light 16 at wavelength λc generated by a continuous wave light source 18 enters SOA 10 from an opposite side.
SOA 10 is essentially a semiconductor laser with antireflection coated facets that amplify the injected continuous wave light 16 by means of stimulated emission. The amplification is based on the XGM phenomenon in which the continuous wave light 16 is cross-gain modulated by the input optical signal 12. Specifically, SOA 10 operates within a region of saturation, in which the amplification gain of the continuous wave light 16 varies inversely with the degree of saturation. Because the degree of saturation increases as the input power of the input optical signal 12 increases, the amplification gain of the continuous wave light 16 varies inversely with the input power of the input optical signal 12. At the same time, because the intensity of the input optical signal 12 is modulated with optical pulses representing input digital data, SOA 10 outputs an output optical signal 20 which is inversely modulated with optical pulses. The output optical signal 20 has the same wavelength λc as the continuous wave light 16, thus effectuating a wavelength conversion from λin to λc. XGM-based wavelength converters shown in FIG. 1 have advantages of low-cost, simplicity and increasing flexibility and bandwidth.
Several problems still exist with prior art XGM-based wavelength converters shown in FIG. 1. One problem is that cross-gain modulation (XGM) inverts the output signal with respect to the input signal. Specifically, a pulse representing a bit “1” in the input signal becomes a bit “0” in the output signal, while a pulse representing a bit “0” in the input signal becomes a bit “1” in the output signal. This inversion often necessitates further data manipulation. Another problem is that XGM exhibits uneven conversion among different wavelengths, and accordingly an XGM-based wavelength converter has an output performance which is wavelength dependent.
FIG. 2 is a diagram showing a wavelength-dependent gain curve associated with a semiconductor optical amplifier operating at wavelength band around 1550 nm. As shown in the curve, the gain is the highest near the center of the band but lower in other regions, and the lowest at two ends (1500 nm and 1590 nm) of the band.
Both the above identified problems are inherent to single cross-gain modulation.